It is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic pituitary adrenal axis and is often produced in response to biological stress (along with its precursor corticotropin-releasing hormone from the hypothalamus). Its principal effects are increased production and release of corticosteroids.
When a pituitary tumor is the cause of elevated ACTH (from the anterior pituitary) this is known as Cushing's Disease and the constellation of signs and symptoms of the excess cortisol (hypercortisolism) is known as Cushing's syndrome. A deficiency of ACTH is a cause of secondary adrenal insufficiency. ACTH is also related to the circadian rhythm in many organisms.
Abellán-Galiana et al. propose an ACTH value <15 pg/mL as a good long-term prognostic marker in the postoperative period of Cushing's Disease. Reaching the ACTH nadir in less time is associated to a lesser recurrence rate 2).